Conventionally, solid electrolyte materials such as yttria doped zirconia (hereinafter, referred to as YSZ) have been used in the applications of solid oxide fuel cells (hereinafter, abbreviated as SOFCs) and the like. SOFCs have higher electric power generation efficiencies and higher discharged thermal energy temperatures than other fuel cells, such as phosphoric acid-type fuel cells and molten carbonate-type fuel cells. Hence, SOFCs have attracted attention as a next-generation type energy-saving electric power generation system.
A basic structure of an SOFC includes a solid electrolyte layer, a fuel electrode layer, and an oxygen electrode layer. When a fuel gas such as hydrogen (H2) flows through and thereby comes into contact with the fuel electrode layer, which faces one surface of the solid electrolyte layer, and an oxidizing agent gas such as the air or oxygen (O2) flows through and thereby comes into contact with the oxygen electrode layer, which faces an opposite surface of the solid electrolyte layer, oxygen ions (O2−) generated in the oxygen electrode layer move through the solid electrolyte layer to the fuel electrode layer, and the O2− react with H2 in the fuel electrode layer. An electric output can be obtained by this electrochemical reaction.
A solid electrolyte material for an SOFC based on such a reaction mechanism needs to have the following characteristics: (1) high oxygen ion conductivity; (2) excellent long-term durability; (3) high material strength; and the like. Particularly from the viewpoint of long-term durability, the most preferred material is YSZ, among zirconia-based solid electrolyte materials.
As the oxygen electrode layer of an SOFC, strontium doped lanthanum manganite (hereinafter, referred to as LSM), strontium doped lanthanum ferrite (hereinafter, referred to as LSF), and strontium and iron doped lanthanum cobaltite (hereinafter, referred to as LSCF) are generally used. A cell is exposed to a high temperature during production of the oxygen electrode layer by the sintering method using any of these materials and during the operation of the cell. Hence, manganese (Mn) in the case of LSM or iron (Fe) in the cases of LSF and LSCF diffuses to YSZ, which is the solid electrolyte layer, and lowers the oxygen ion conductivity. To suppress the diffusion, a solid electrolyte layer of YSZ containing alumina has been proposed (see Japanese Patent Application Publication No. Hei 11-354139). Note that Japanese Patent Application Publication No. Hei 11-354139 does not describe the diffusion suppression effect. However, since a separately invented solid electrolyte layer made of scandia doped zirconia containing alumina is described to have the diffusion suppression effect, also the solid electrolyte layer of YSZ containing alumina presumably achieves the same effect (see Japanese Patent Application Publication No. Hei 8-250135).